More
on Garter Snakes

NARCISSE, CanadaOne
of nature's strangest spectacles happens every spring in southern Manitoba.
Tens of thousands of red-sided garter snakes emerge from their underground
hibernation dens and engage in mating balls.

The male red-sided garters
emerge first and wait patiently for the females to follow. Each time a
female appears, the males surround her. The males look like a mass of
living spaghetti, said Robert Mason, a zoologist at Oregon
State University. The ball of snakes will writhe and sometimes even roll
over land, until one male finally mates with the female.

The weirdest part of
the spectacle occurs when some male red-sided garters impersonate females
and find themselves at the center of a mating ball. These she-male
snakes fool the males by exuding a female pheromonea chemical released
by one animal that affects another animal. The female pheromone fools
male red-siders into thinking the she-male is a female.

Why the she-males act
out this charade has long been a mystery. Now Mason and Rick Shine, a
biologist from the University of Sydney in Australia, think they've found
the answer.

All snakes are ectothermic,
or cold-blooded. When a garter snake emerges from the cold ground, it
is cold and lacking in energy until it's warmed by the sun. In such a
state, the snake is vulnerable to attacks by predators. Previous studies
have shown that she-male garters are slower and weaker than male garters
and, hence, even more vulnerable.

The she-male's charade
has two purposes, say the scientists. One: It warms the she-male garter
like a blanket; and two: It surrounds him-her with a phalanx of bodyguards.

The annual red-garter
mating balls are a big tourist attraction in Manitobaand a source
of many tales. One unsuspecting couple built a house on top of an empty
snake pit one summer, only to find their property swarmed by thousands
of red-sided garters returning to their traditional hibernation den in
the fall. The couple quickly relocated their new house.

Male garter snakes limping
out of hibernation in northern Manitoba can mimic females and drive dozens
of other guys to wriggle over them. The force behind this deluded orgy
may not be sex, though.

Until now, scientists
presumed that female mimicry gives its perpetrators an edge in mating,
explains Rick Shine of the University of Sydney in Australia. But there's
no evidence for any mating advantages for the "she-male" garter
snakes, nor do scientists know how these awakening snakes attract other
males.

The fakery needs a new
explanation, argue Shine and his colleagues in the Nov. 15 Nature.
They propose that these snakes creeping out of 8 months of chilled inactivity
find that the ball of suitors provides body heat and protection from crows
and other birds.

"If you're weak
and slow and cold, what you want is a whole bunch of warm snakes on top
of you," says Shine.

The animals observed
in the new study belong to a subspecies of the garter snake found across
much of North America. In Manitoba, garter snakes converge on the few
spots suitable for hibernation without freezing. In spring, males emerge
and wait for the sporadic rousing of females. "You can have 25,000
to 30,000 snakes in a den the size of an average living room," says
Robert T. Mason of Oregon State University in Corvallis, a coauthor of
the new study. When a female slides by, up to 100 males knot around her.
She permits just one to mate.

Mason and a colleague
first described a she-male mating ball in 1985, but scientists are still
searching for its benefits.

To check heat transfer,
the researchers monitored female snakes that birds had killed. Males courted
the corpses, often heating them more than 3°C. In a temperature test
with live females that started at 4°C, those courted in a mating ball
warmed to 20°C faster than did those separated from any suitors.

The researchers also
explored recovery from hibernation. They caught newly emerged males that
attracted male attention. The she-males that researchers warmed to 28°C
turned into regular guys within 3 hours, but those at 10°C still inspired
courtship after 5 hours. This season, Mason hopes to check just-emerging
male snakes for pheromones.

When the snakes emerge,
birds gather and kill hundreds, say the investigators. Garter snakes have
no venom and can only flee to defend themselves. In sprint tests, however,
cold snakes move slowly. A courtship tangle could protect the insiders,
the researchers propose.

"Female mimicry
is pretty common," says Stephen M. Shuster of Northern Arizona University
in Flagstaff. Some animals show a clear mating benefit from the deception.
In a pill bug relative that Shuster studies, males with antlers on their
rears defend cavities where females gather. Occasionally, a male with
no antlers and the domed body shape typical of females flirts with the
defender, enters the cavity unchallenged, and sires up to 60 percent of
the females' offspring.

Barry R. Sinervo of
the University of California, Santa Cruz sees mating advantages to female
mimicry among side-blotched lizards. For she-male snakes, though, he calls
the heat-and-safety payoff "a plausible idea" and predicts that
researchers will consider it for other species. He says, "It really
takes just one example, then people start looking more closely."

In northern Manitoba,
deluded male garter snakes swarm over another male (arrow) still dusty
and chilled from hibernation. Eager males court the newcomer regardless
of its failure to mate.